1. Field of the Invention
The invention generally relates to a pulse width modulation (PWM) controller for a switching voltage regulator, and more particularly, relates to a feed-forward circuit that adjusts an output pulse width inversely with respect to a supply voltage to compensate for transients or changes in the supply voltage.
2. Description of the Related Art
A switching voltage regulator generally accepts a direct current (DC) supply voltage at a first level and generates a regulated output voltage at a second level. The second level can be higher or lower than the first level. Different sources, such as a battery or an alternating current (AC) adapter, can provide the DC supply voltage. The different sources can provide different DC supply voltage levels, and the DC supply voltage level can vary over time for the same source. In a PWM-based switching voltage regulator, the regulated output voltage tends to increase or decrease proportionally with an increase or a decrease in the DC supply voltage. A PWM controller for the switching voltage regulator typically includes a feedback loop that forces the regulated output voltage back to a nominal level. However, the feedback loop is relatively slow for stable operations and does not usually react fast enough to rapid changes (or transients) in the DC supply voltage to prevent overshoots or undershoots in the regulated output voltage.
A feed-forward circuit can be used to compensate for transients in the DC supply voltage. One type of feed-forward circuits provides compensation for changes in the DC supply voltage by changing a slope of a ramp signal in the PWM controller to force a change in a pulse width of a PWM output that controls the regulated output voltage level. There is generally significant delay in this type of pulse width correction since the slope of the ramp signal is generally controlled by a current charging a capacitor, and voltage across a capacitor cannot change instantaneously. For example, a practical PWM ramp generator uses a current source or resistor to charge a capacitor to produce a ramp voltage across the capacitor. The ramp voltage is compared with a control signal from an error amplifier to modulate a pulse width of a PWM output. Since the ramp voltage (i.e., the voltage across the capacitor) cannot change instantaneously when charging current changes, the ramp voltage cannot step instantaneously to follow and correct for a fast supply voltage transient. Correction may be delayed by a minimum of one half cycle and up to several cycles of the ramp voltage.